The amino acid sequences of the two subunits of ovine lutropin, designated alpha and beta, have been established independently by Liu et al. and Sairam et al. In a previous paper we present the location of disulfide bridges in the lutropin-alpha molecule. We have now succeeded in proposing the disulfide linkages in the beta subunit of ovine lutropin. Peptide fragments were obtained from partial acid and enzymic hydrolysis of lutropin-beta. Results indicate that the disulfide bridges are formed by residues 9-38, 23-72, 26-110, 34-90, 57-88, and 93-100. When one ml of antiserum to rat prolactin was administered daily beginning on day 6 and continued through day 8, no deleterious effects on the course of the pregnancy in the hamster were noticed; also, there was no reduction in the ovarian or mammary gland weight. Even when daily injections (1ml) of the antiserum were extended to day 14, no drastic effects on the course of pregnancy occurred; however, three of the five hamsters showed slight external vaginal bleeding on day 10 of pregnancy, which later subsided. On autopsy, all the animals had a few resorbed sites, but out of a total of 45 sites, only 12 were resorbed with mean ovarian weight comparable to that of the normal rabbit serum treated group. A significant reduction in the mammary gland weight was also noticed. In contrast to the above observations, neutralization of endogenous lutropin for two days by daily injections of 0.5 ml antiserum to ovine lutropin resulted in complete termination of pregnancy with a drastic reduction in ovarian weight. From these data, it may be concluded that prolactin does not play a major role in maintaining pregnancy of the hamster. BIBLIOGRAPHIC REFERENCES: Farmer, S.W., Clarke, W.C., Papkoff, H., Nishioka, R.S., Bern, H.A. and Li, C.H. Studies on the purification and properties of teleost prolactin. Life Sciences 16, 149-158 (1975). Bewley, T.A. and Li, C.H. Studies of pituitary lactogenic hormone. Physico-chemical characteristics of porcine prolactin. Arch. Biochem. Biophys. 167, 80-90 (1975).